Dopant profiles in heavily doped ZnO

X-ray photoelectron spectroscopy (XPS) is used to compare the composition as a function of depth of as-grown ZnO films heavily doped with Ga and similar samples annealed in air for 10 min. at 600 °C, with particular attention given to the near-surface region. These films were grown by pulsed laser deposition (PLD) using a ZnO target containing 3 wt% Ga2O3. The electrical properties of these samples were determined from temperature-dependent Hall-effect measurements. The as-grown film has the following characteristics: i) a ∼1:1 Zn:O ratio with a Ga concentration of ∼ 3.3 atomic percent; ii) no excess Ga in the near-surface region; and iii) excellent electrical characteristics: ρ=2.42×10-4 Ω-cm, n=8.05×1020 cm-3, and μ=32.1 cm2/V-s at 300 K. For the annealed sample: i) the Zn:O ratio remains ∼ 1:1, but the Ga concentration is ∼ 3 atomic percent which is ∼10% lower than in the as-grown film; ii) ∼7 at% Ga is measured in the near-surface region; and iii) a significant increase in resistivity to ρ = 0.99 Ω-cm, n = 1.97×1018 cm-3, and μ=3.2 cm2/V-s at 300 K. Analysis of the O chemical shift suggests formation of a mixed ZnO/Ga2O3 surface layer ≤ 5 nm thick accounts for the observed changes in the Ga profile after annealing.